Clean pipes are classified, according to the method of the production thereof, into the bright annealing finish type in which a stainless steel pipe after being cold drawn is subjected to bright annealing treatment; the electric polishing finish type (EP type) in which the inner surface of a stainless steel pipe of a bright annealing finish is further smoothed by means of electrochemical polishing; and like method.
It is well known that the inner surface roughness of a clean pipe is closely related to the production of impurities or fine particles and the discharge of water vapor from the inner surface of the pipe. In an apparatus which is required to have a high degree of cleanliness, clean pipes of the electrolytic polishing finish type whose inner surface roughness becomes lower and are expensive, are used.
In the production of a pipe having a smooth inner surface, there has been a conventional method employed in which a tubing material is subjected to cold plug drawing. Cold plug drawing is a method of processing in which a tubing material 85 is cold drawn with the outer and inner surfaces thereof constrained, as shown in FIG. 10, by a fixing die 86 having a round hole and a plug 81, and the outlet-side end of the tubing material 85 chucked (not illustrated). A chemical conversion treatment lubrication and oil lubrication are the general methods of lubrication between the tools (the die 86 and the plug 81) and the tubing material 85, and oil lubrication capable of forming a thin lubricating film is employed in order to obtain highly-smoothed inner and outer surfaces.
A material for a pipe such as a clean pipe which is required to have a more highly-smoothed inner surface is subjected to a highly-smoothing treatment such as electrochemical polishing after it is cold drawn by the above-described method.
Typically, there are two types of methods for cold drawing in which different plugs are used as shown in FIG. 10.
FIG. 10(a) shows a method in which a cylindrical plug 81 whose outside diameter is uniform is used. The cylindrical plug 81 is connected with a plug-supporting rod 87. This method is used for producing pipes of relatively large dimensions.
FIG. 10(b) shows a method in which a floating plug 82 is used. This method is characterized by the shape of the plug and by the method for supporting the plug. As illustrated in this figure, the floating plug 82 is tapered, and the cone angle 2.beta. of the plug is smaller than the facial angle 2.alpha. of the die.
For this reason, those forces which act on the floating plug 82 are the frictional force which acts in the direction of drawing, and, in addition to this, the pushing-back force which acts on the tapered surface of the plug in the direction reverse to the direction of drawing. The frictional force and the pushing-back force are canceled and balanced with each other. Therefore, such a plug-supporting rod 87 as is used in the method using a cylindrical plug shown in FIG. 10(a) is not needed, and even if a supporting rod is provided in consideration of operation, almost no force acts on the supporting rod.
Since the floating plug has the above-described characteristics, the method using this plug is commonly adopted to draw a tubing material to obtain, in particular, a pipe whose diameter is small. However, in the case where this plug is used, the balancing position of the plug varies depending upon the state of lubricating films provided on the inner and outer surfaces of the tubing material, or upon the force for drawing the tubing material. Since the change of the balancing position of the plug is extremely obstructive to the operation for drawing the tubing material, proposals for improvements for this change have been made. For instance, Japanese Laid-Open Patent Publication No. 72419/1988 discloses a method in which a plug is maintained at a proper position on a die by balancing the frictional force and the pushing-back force which act, during the process of drawing, on the horizontal surface and the tapered surface of the plug, respectively.
FIG. 11 is a longitudinal section explaining the shape of the plug which is used for drawing a tubing material in the above-described method, and the method of drawing. As illustrated in the figure, a plug 111 has a horizontal surface 112 which provides a uniform outside diameter to the plug, a first tapered surface 113 with which the outside diameter of the plug is decreased toward the direction opposite to the direction of drawing, and a second tapered surface 114 which is continued to the first tapered surface and with which the outside diameter of the plug is increased. Therefore, the forces which act on the horizontal surface 112 and on the second tapered surface 114 when a tubing material is drawn can be balanced, and the plug 111 can thus be maintained at a proper position on the die 115.
The minimum inner surface roughness expressed in Rmax of a pipe obtained by means of the above described conventional cold plug drawing is limited to approximately 1.1 micrometers, for example, when a clean pipe having an outside diameter of 6 mm and a wall thickness of 1 mm is produced by using SUS 304, and it is difficult to make the Rmax lower than this limit. Further, even in the method disclosed in Japanese Laid-Open Patent Publication No. 72419/1988, it is necessary to make the difference between the outside diameters on the inlet side and on the outlet side of the part with the first tapered surface 113 as considerably large as several-tenths mm. When the difference between the outside diameters of the part with the first tapered surface 113 is large, seizure is caused when ordinary oil lubrication is conducted, so that it is necessary to conduct chemical conversion treatment lubrication which is excellent in antiseizure properties. However, as will be described later, when chemical conversion treatment lubrication is conducted, the lubricating film produced is thick, so that a tubing material after being subjected to drawing will have a high inner surface roughness. It is thus impossible to make the inner surface roughness expressed in Rmax to 1.0 micrometer or less even when squeezing is conducted. Therefore, in order to obtain a pipe which is required to have an inner surface roughness expressed in Rmax of 1.0 micrometer or less, it is essential, as mentioned above, to conduct a highly-smoothing treatment such as electrochemical polishing after cold plug drawing is conducted. As a result, the price of the final product becomes extremely high, approximately four times the price of a pipe produced by conventional cold plug drawing.
Some techniques have been known as methods in which a plug of a specific shape is used when cold plug drawing is conducted in order to bring about special effects on the inner surface of a pipe.
For instance, Japanese Patent Publication No. 7244/1987 discloses a method in which a tubing material is processed by using a plug of a specific shape to form a work-hardened layer on the inner surface of a pipe so as to prevent the stainless steel pipe from being oxidized by water vapor.
FIG. 12(a-c) include a side view and longitudinal sections which show the shape of a plug having a protruding portion, used for the above-described processing, and a method of drawing, using the plug. FIG. 12(a) and FIG. 12(b) are illustrations showing the process of drawing, and FIG. 12(c) is a side view of the plug. Shown in this figure is a method in which a work-hardened layer is formed on the inner surface of a tubing material 95 by increasing the inside diameter of the tubing material 95 by the use of plug 91 provided with a protruding portion 94 and a plug-supporting rod 97 as illustrated in the figure. The object of this method is to form a work-hardened layer, and the inner surface roughness obtained, expressed in Rmax is extremely high from 18 to 25 micrometers. Further, in this method, the tubing material 95 is processed without constraining the outer surface thereof, so that it is impossible to obtain 1 micrometer or less of the inner surface roughness expressed in Rmax as in a part of the examples which will be described later.
A method shown in FIG. 13 has also been used as another conventional method of processing. FIG. 13 is a side view, partly in cross section, and a longitudinal section which show the shape of a conventional plug used for increasing the inside diameter of a pipe, and a method of drawing, using the plug. The object of this method is to improve the dimensional accuracy of the inner surface of a pipe such as a steel pipe for a cylinder (in particular, the roundness of the inside of a pipe).
As shown in FIG. 13, plug 101 connected with a supporting rod 107 is so tapered that the diameter of the plug is slightly increased toward the outlet side of drawing. The inside diameter of a tubing material 105 is increased by this plug 101 and a die 106, and, at the same time, the roundness of the inside of the tubing material 105 is improved. However, since the increase of the inside diameter is slightly in this method, the effect of squeezing the inner surface of the tubing material, which will be described later, is small, that is, only a thin layer of shear plastic deformation is formed on the inner surface of the tubing material. It is therefore impossible to obtain a highly-smoothed inner surface which is required for a clean pipe.
As illustrated in the figure, the diameter of the plug 101 is slightly increased toward the outlet side. However, with respect to the wall-thickness-processed part of the tubing material 105, there is no great difference between it and that part of the tubing material obtained by using the conventional plug as shown in FIG. 10(a) and FIG. 10(b). Therefore, although the roundness of the inside of a pipe can be improved by the plug of this shape, the inner surface roughness of a pipe cannot be improved.